We have been studying inflammation and immunity in the Syrian hamster. A sex limited serum protein of females called Female Protein (FP) was shown to be functionally similar to other homologous acute phase proteins, C-reactive protein (CRP) and serum amyloid P component (SAP). FP binds complement when reacted with poly-L-arginine (similar to CRP) and is found in hamster amyloid deposits (similar to SAP). The acute phase response consists of a decrease in the normally high levels of serum FP in female hamsters in contrast to the typical increase serum levels of SAP and CRP seen in other mammals and also FP in serum of male hamsters. In regard to hamster immune response, the humoral and cellular immune response of the hamster spleen to soluble protein antigens (but not particulate antigen) was found deficient. A population of normal spleen cells (nylon wool nonadherent) added to sensitized lymph node cells, markedly suppressed the antigen specific proliferation. Numerous IgG1 and IgG2 antibody forming precursors were detectable in spleen cells after in vitro culture of sensitized spleen cells incubated in the absence of antigen. Many antibody forming cells were found in spleens of recipients reconstituted with sensitized lymph node cells.